Alkali-metal vanadium sulfate beta alums, MV(SO4)(2)center dot 12H(2)O (M=K, Rb or Cs): Structural anomalies related to unsymmetrical occupancy of the t(2g) (O-h) orbitals

Crystal structure determinations of the alums MV(SO4)(2) . 12H(2)O (M = K, Rb or Cs) showed that they all belong to the beta structural class. This observation, while predicted earlier from the crystal morphology, is not expected from trends based on the sizes of the monovalent cations and results in formal 12-co-ordination for K+ and Rb+. The structures were determined at 120 (K) and 295 (Rb and Cs) K and refined to R values of 0.025, 0.040 and 0.022 for 877, 348 and 2008 independent 'observed' reflections, respectively. The crystals are cubic, space group Pa (3) over bar with a = 12.253(5), 12.367(2) and 12.434(1) Angstrom, respectively. Despite the relatively imprecise determination of the hydrogen-atom positions by X-ray diffraction, analysis of the available neutron structures indicates that the water co-ordination geometry may be deduced with good precision from the heavy-atom coordinates. On this basis the stereochemistry of water co-ordination to M(III) is conserved in the three structures. The occurrence of the beta structure for these salts is likely to be due to a preferred stereochemistry for water co-ordination to vanadium(III) which arises from lifting of the degeneracy of the T-3(2g)(O-h) ground term.